1. Field of the Invention
The present invention relates to an inverter control device based on instruction current follower control capable of controlling the operation of an inverter having a three phase bridge circuit by selecting an optimum switching vector so that a detected current value of a coil winding of each phase follows a given instruction current value.
2. Description of the Related Art
There have been various inverter control devices for controlling the operation of an inverter that drives an electric motor and an electric generator. FIG. 12A is a block diagram showing a configuration of an inverter to be controlled by an inverter control device of a related art. FIG. 12B is a block diagram showing a circuit configuration of the inverter control circuit of the related art. The inverter control device based on instruction current follower control is well known as one of the control devices. Such an inverter control device of a related art has a configuration shown in FIG. 12B.
The inverter control device shown in FIG. 12B selects an optimum switching vector so that a detected current value follows a given instruction current value so as to control the operation of the inverter shown in FIG. 12A, The inverter shown in FIG. 12A is made of a three phase bridge circuit. In FIG. 12A, reference character “MG” means an electric motor and an electric generator. For brevity, the term “the electric motor MG” will be used for both the electric motor and the electric generator through the following description.
First, the inverter 101 to be controlled, as the target in control, has a direct current (DC) power supply 103, a smoothing capacitor 105, a three phase bridge circuit 107, and a current detection circuit 109.
The smoothing capacitor 105 smoothes the voltage of the DC power supply 103 to be supplied to the three phase bridge circuit 107. The three phase bridge circuit 107 receives the electric power from the DC power supply 103 through the power source lines and supplies the current to the coil winding of each of the phases U, V, and W of the electric motor MG. The current detection circuit 109 detects the magnitude of the current supplied from the three phase bridge circuit 107 to the coil winding of each of the phases U, V, and W of the electric motor MG.
The switching elements SWa and SWb are connected in series to make a pair, the switching elements SWc and SWd are connected in series to make a pair, and the switching elements SWe and SWf are connected in series to make a pair.
The three phase bridge circuit 107 has six reflux current diodes and the six switching elements SWa to SWf. Each switching element and the corresponding reflux diode are connected in inversely parallel connection. Three pairs of the switching elements SWx (x=a, b, c, d, e, and f) are arranged in parallel to each other. Between the positive electrode and the negative electrode of the DC power supply 103, the three pairs of the switching elements SWx are connected in parallel. Each pair of the switching elements is connected to the coil winding of each of the corresponding phases U, V, and W of the electric motor, respectively.
Next, the inverter control device 110 has a current deviation detection section 111, a switching vector determination section 113, and a driving signal generation section 117. The current deviation detection section 111 detects deviations between the detection current values and the current instruction values given in advance for each phase, and outputs detection signals du, dv, and dw that indicate the deviations (as current deviation) between the current values iu, iv, and iw detected by the current detection circuit 109 and current instruction values iu*, iv*, and iw*, respectively.
The switching vector determination section 113 determines a switching vector SV according to the deviation signals du, dv, and dw detected by the current deviation detection section 111. The driving signal generation section 117 generates driving signals UP, UN, VP, VN, WP, and WN with which the switching elements SWa to SWf forming the three phase bridge circuit 107 are switched according to the switching vector SV determined by the switching vector determination section 113.
Reference character XP designates the driving signals to be transferred to the switching elements SWp (p=a, c, and e, namely SWa, SWc, and SWe) connected to the positive electrode of the X phase (X=U, V, and W) of the electric motor MG. Reference character XN designates the driving signals to be transferred to the switching elements SWn (n=b, d, and f, namely SWb, SWd, and SWf) connected to the negative electrode of the X phase (X=U, V, and W) of the electric motor MG.
The driving signal generation section 117 generates the driving signals that set one switching element to ON and the other switching element to OFF in each pair of the switching element SWp and SWn (namely, in each of the pair of SWa and SWb, the pair of SWc and SWd, and the pair of SWe and SWf). Further, the driving signal generation section 117 generates the driving signals that set both the switching elements in each pair to OFF (as a dead time) when the ON/OFF state is switched.
The switching vector SV represents an arrangement of switching state of the switching elements SW corresponding to the phases U, V, and W, where reference number “1” indicates ON state of the switching element SWp (p=a, c, and e) connected to the positive electrode of the X phase (X=U, V, and W) of the electric motor MG, and reference number “0” denotes ON state of the switching element SWn (n=b, d, and f) connected to the negative electrode of the X phase (X=U, V, and W) of the electric motor MG. That is, the switching vector SV represents eights states (=23).
On driving the inverter 101 under the control of the switching vector SV, the amount of current increases in the phase of the electric motor MG under the ON state “1” and decreases in the phase of the electric motor MG under the OFF state “0”. In particular, on driving the inverter 101 under the switching vector SV designated by the states “000” and “111” (hereinafter, referred to as “zero vector”), the change of current in each phase is suppressed because there is no voltage difference between the three phases U, W, and W of the electric motor MG.
In order to avoid any occurrence of unnecessary switching caused by a noise and the like on deciding whether or not the switching vector SV is changed, the inverter control device 110 uses a decision threshold value involving a given hysteresis for deciding the magnitude of a deviation. The inverter control device 101 controls or selects the switching vector so that the current is decreased when a current value detected is larger than an instruction value by ΔI, and on the contrary, the current is changed by increasing the current when the current value detected is smaller than an instruction value by ΔI.
In general, it is well known that the number of switching is increased in a low rotational speed of the electric motor MG by such an inverter control device based on instruction current follower control. This means that the magnitude of the currents such as the currents iu, iv, and iw, flowing through the coil windings of each phase to be driven, are greatly changed to the switching vectors other than the zero vectors “000” and “111” when the rotational speed of the electric motor MG is low and generates a small counter electromotive force. On the contrary, the current instruction values iu*, iv* and iw* take a low frequency waveform of an extremely low change rate. Accordingly, the change rate of the detected current values iu, iv, and iw detected when the ON and OFF of the switching elements SWp and SWn are switched is over the hysteresis width ΔI of the decision threshold value in a moment. As a result, because an excessive large current or an excessive small current are switched in a short time period, the number of the switching is increased.
Increasing the number of switching introduces the increase of switching loss and accordingly because large rated elements must be used as the switching elements SWa to SWf, the degree of freedom in design for the inverter 101 is thereby limited. In fact, it is possible to decrease the number of the switching slightly when the hysterisys width ΔI of the decision threshold value is set to a large value, this causes a problem of increasing a current distortion flowing in the coil windings of each phase of the electric motor MG to be driven.
Further, because the inverter control device involves a drawback to increases the switching loss at a low revolution speed of the electric motor MG, it is difficult to apply such an inverter to the application in which the electric motor operates in a wide range from a low revolution to a high revolution such as an electric motor that is a power source for a hybrid vehicle.
In order to avoid the conventional problem, the Japanese patent laid open publication NO. 2000-316284 as one of conventional well known techniques has disclosed a conventional inverter control device based on a current control in which a current deviation vector using a current deviation selects a zero vector when it is within an error range determined according to current control precision.
Thus, the active use of the zero vector may eliminate the conventional problem described above because it can suppress an excess control to the current and decrease the number of the switching.
In order to make such an inverter control device cheaply, the recent trend is to incorporate in the inverter control device a microprocessor so as to perform digital control for various processes of each configuration element forming the inverter control device. In this case, the response of the inverter control device equipped with a microprocessor is limited by the control period of the microprocessor, in particular, by a sampling period of an analogue to digital (A/D) converter that converts an analogue current value detected to a digital current value. On making the inverter control device cheaply, the microprocessor with a sampling period of several μs is installed in the inverter control device.
However, the conventional technique disclosed in the Japanese patent laid open publication NO. 2000-316284 can not select the zero vector in a low revolution speed of the electric motor MG and as a result it cannot decrease the number of the switching.
In other words, because the current change is increased during the low revolution of the electric motor, it is impossible to detect the state in which the current deviation is within an error range and as a result, the zero vector is not selected.